1. Technical Field
This application relates to an apparatus and method for determining the probability of patient burn during electrosurgery, and more particularly to determining the probability of patient burn under a return electrode in a monopolar electrosurgical system.
2. Background of Related Art
During electrosurgery, a source or active electrode delivers energy, such as radio frequency energy, to the patient and a return electrode carries the current back to the electrosurgical generator. In monopolar electrosurgery, the source electrode is typically the hand-held instrument placed by the surgeon at the surgical site and the high current density flow at this electrode creates the desired surgical effect of cutting or coagulating tissue. The patient return electrode is placed at a remote site from the source electrode and is typically in the form of a pad adhesively adhered to the patient.
The return electrode has a large patient contact surface area to minimize heating at that site since the smaller the surface area, the greater the current density and the greater the intensity of the heat. That is, the area of the return electrode that is adhered to the patient is important because it is the current density of the electrical signal that heats the tissue. A larger surface contact area is desirable to reduce heat intensity. Return electrodes are sized based on assumptions of the maximum current seen in surgery and the duty cycle (the percentage of time the generator is on) during the procedure. The first types of return electrodes were in the form of large metal plates covered with conductive jelly. Later, adhesive electrodes were developed with a single metal foil covered with conductive jelly or conductive adhesive. However, one problem with these adhesive electrodes was that if a portion peeled from the patient, the contact area of the electrode with the patient decreased, thereby increasing the current density at the adhered portion and in turn increasing the heat applied to the tissue. This risked burning the patients in the area under the adhered portion of the return electrode if the tissue was heated beyond the point where the circulation could cool the skin.
To address this problem, split return electrodes and hardware circuits, generically called Return Electrode Contact Quality Monitors (RECQMs), were developed. These split electrodes consist of two separate conductive foils. The hardware circuit uses an AC signal between the two electrode halves to measure the impedance therebetween. This impedance measurement is indicative of how well the return electrode is adhered to the patient since the impedance between the two halves is directly related to the area of patient contact. That is, if the electrode begins to peel from the patient, the impedance increases since the contact area of the electrode decreases. Current RECQMs are designed to sense this change in impedance so that when the percentage increase in impedance exceeds a predetermined value or the measured impedance exceeds a threshold level, the electrosurgical generator is shut down to reduce the chances of burning the patient
Although monitoring circuits in present use are effective, they do not take into account the amount of time the current is being delivered. As new surgical procedures continue to be developed that utilize higher current and higher duty cycles, increased heating of tissue under the return electrode will occur. It would therefore be advantageous to design a monitoring circuit which would also factor in the amount of time the current is being delivered in determining the probability of a patient burn. Based on this probability determination, an alarm signal can be generated or power supplied from the generator can be shut off.
U.S. Pat. No. 4,657,015 discloses a control device for cutting off high frequency current during electrosurgery if the heat buildup in the body tissue exceeds a predetermined value. In the '015 patent, a control electrode is affixed to the body spaced from the active electrode and separate from the neutral (return) electrode. The control electrode is designed to pick up the voltage existing on the body. The voltage signal is squared, integrated over time and compared to a reference voltage. The high frequency generator is turned off if the voltage value exceeds the reference voltage. The '015 patent does not effectively measure heating under the return electrode since the measurements are calculated by a separate control electrode. The '105 patent even states that the effective surface area of the neutral electrode is not a factor in the heat calculations. Also, the amount of time the energy is being applied is not a factor in the heat calculations. Additionally, the '015 patent uses voltage measurement to determine overheating of tissue. It is currently believed by the inventors of this application that current measurement provides a more accurate parameter because voltage values actually measure the ability to transfer energy through the tissue while current values measure actual heating of the tissue.
U.S. Pat. No. 4,741,334 discloses a control circuit intended to reduce burning of tissue. As in the '015 patent, a separate control electrode is provided to determine the body voltage. The control electrode is spaced from the neutral electrode and functions to detect a high frequency body surface voltage. The body surface voltage is converted into dc voltage by a converter and inputted to a comparator for comparison to a reference voltage. The generator is turned off if the body voltage exceeds the reference voltage. The '015 patent also discloses a monitor circuit for testing whether the neutral electrode is in good contact with the body surface of the patient. A comparator compares the body surface voltage detected by the control electrode with a reference voltage derived from the operational voltage of the surgical device. An audible signal is produced when these voltage values reach a predetermined ratio. Similar to the '015 patent, the '334 patent requires an additional electrode, measures voltage instead of current to determine overheating, and does not factor in the amount of time the high frequency energy is being applied.
As noted above, it would be advantageous to provide a monitoring circuit which effectively determines the probability of overheating tissue, i.e. the probability of patient burn, by measuring current and factoring in the time period of energy delivery of energy.